Tumors contain immune system cells that attack cancer cells but are present in very small numbers, leading to the hope that a flood of these antitumor cells might eradicate cancer. This approach can work in melanomas, but has not yet worked against other kinds of cancers. Now, there's a better way to concentrate antitumor immune system cells: they have a protein called CD137 on their surfaces; researchers recently used this distinction to extract these antitumor cells quickly and easily from both melanomas and ovarian cancer. The new study showed that peoples' own antitumor cells recognized their tumors and that injections of these cancer fighting cells kept tumors from growing in mice. If other types of cancer also contain these antitumor immune system cells, this new approach could hold promise for treating a wide range of tumors.